Six hundred and sixty serum samples from 135 patients treated with human leukocyte alpha interferon (Hu IFN (Le) (New York Blood Center, Melville, N.Y.) for Juvenile Laryngeal Papillomatosis (J.L.P.) were tested for the presence of neutralizing antibodies to the above-mentioned alpha interferon. Eleven samples were found to contain neutralizing antibodies to alpha interferon from the N.Y.B.C. Ten of these contained neutralizing antibodies to human leukocyte ` interferon (Alferon; Interferon Sciences, New Brunswick, N.J.) and nine contained neutralizing antibodies to recombinant ` interferon (Intron A, Schering-Plough, Kenilworth, N.J.). Preliminary data suggest that the eleven neutralization positive samples are all positive in ELISA assays for binding to the same types of interferon which they neutralize but, of course, these assays must be repeated. Also of interest is whether any of the positive samples cross react with any of the human lymphoblastoid components which are purified in our laboratory. Neutralization assays will be performed using the eluates from monoclonal antibody columns from which these components are purified. If any neutralization is seen, the individual components found in that eluate will be assayed.